This invention relates to communication networks, and in particular, to methods and systems for ensuring the integrity of data transmission in the event of an equipment failure within the network.
A communication network typically includes a large number of nodes connected by transmission lines. In a modem network, these transmission lines are often optical fibers. Such fibers are extremely thin and therefore susceptible to mechanical breakage. In addition, because fibers are so thin, the alignment between fibers at a junction must be extremely precise. These junctions are therefore easily disrupted by mechanical shock or vibration. Even slight kinks or bends in a fiber can cause internal reflections that lead to significant degradation in signal quality.
Although every attempt is made to isolate a fiber from mechanical disturbance, it is difficult to reliably do so. Buried fibers routinely fall prey to backhoes in construction accidents. Over the years, the accumulated effect of the vibration of passing subway trains can gradually degrade communication. Not all disruptions result from human activity, however. Even a minor earthquake can cause isolated disruptions in service.
A network can also fail as a result of disruption within a node. For example, the laser at the transmitting end of each fiber can gradually deteriorate. Since nodes can include complex electronic systems, they too are subject to failure from a variety of causes.
To avoid excessive service disruption in the event of network failure, it is desirable to provide the network with redundancy. One method of achieving this is to arrange the nodes of a communication network in a ring and to connect the nodes with both two independent fibers: a working fiber and a protection fiber. A ring connected in this way is referred to in the art as a UPSR (Unidirectional Path Switched Ring).
In a UPSR, a source node transmits two copies of a data frame to a destination node. A working copy of the data frame travels clockwise around the ring on the working fiber and a protection copy of the frame travels counter-clockwise around the ring on the protection fiber. If the destination node finds that the protection copy matches the working copy, it accepts the working copy. Otherwise, the destination node selects the better of the two copies.
As it makes its way to the destination node from the source node, a data frame can pass through many other nodes. In these intervening nodes, there may be data packets queued for transmission on the ring. In addition, there may be space within the data frame for accommodating some of these data packets. Because these empty spaces represent a waste of network resources, it would be useful to accommodate some of these queued data packets in those spaces.
Unfortunately, as soon as the data frame accepts a data packet from a node other than the source node, the working copy of the data frame will inevitably differ from the protection copy of the frame. Thus, upon comparing the working copy with the protection copy, the destination node will receive two different frames with no way to determine whether the difference is the result of additional data on the frame or a disruption in transmission.
A communication network according to the invention circumvents the foregoing difficulties by providing nodes that do not rely on a comparison between two copies of a data frame in order to detect the existence of an error. Instead, each node adopts a signaling protocol that informs all the other nodes in the network of the condition of the signals arriving at that node from an adjacent node. In response to these signals, each node makes an independent decision as to whether to bypass its adjacent nodes on the network.
The communication network provides a method for reconfiguring a ring having a plurality of nodes connected by first and second channels. Examples of such rings include SONET (Synchronous Optical Network) rings and WDM (Wavelength Division Multiplexing) rings.
When a disruption occurs, there will be a first node and a second node adjacent to, and on either side of, the disruption. Upon the detection of the disruption, the first node signals each of the other nodes to cause that other node to determine if it is the second node, and, if so, to identify itself as such. If it is not, that node continues to operate in its normal mode. However, if that node determines that it is the second node, it sends an acknowledgement signal back toward the first node and forms a bridge between the first and second channels, thereby preventing data from proceeding further toward the disruption. Upon receipt of the acknowledgement, the first node likewise forms a bridge between the first and second channels, thereby preventing data from proceeding further toward the disruption. This results in the isolation of that disruption and the combination of the first and second channels to form a new ring that excludes the disruption.
In one aspect of the invention, the first node sends, by way of the first channel, a first fault signal indicative of a signal fault on the first channel. A second node monitors the second channel for information indicative of the signal fault. On the basis of this information and the first fault signal, the second node forms a first bridge and thereby disconnects a portion of the ring. In addition, the second node sends an acknowledgement signal, by way of the second channel, to the first node.
The information indicative of the signal fault can be a second fault signal. However, it can also be loss of signal on the second channel. This feature permits the data protection to function correctly when both the working channel and the protection channel are disrupted.
In response to the acknowledgement signal, the first node forms a second bridge, thereby disconnecting another portion of the ring. This results in a reconfigured ring in which no signal faults are present in either the first or the second channel.
In a typical communication network, there can be several intervening nodes on the first and second channels connecting the first node and the second node. The method of the invention can thus include routing the acknowledgement signal through a third node selected from the plurality of nodes forming the network.
Forming the first bridge can include directing data traffic arriving at the first node by way of the second channel out through the first channel. This is preferably accompanied by forming the second bridge by directing inbound traffic arriving at the second node outbound on the second fiber.
The method can also include detecting a signal fault on the first channel. The signal fault can be a loss of a signal on the first channel or a degradation of the signal on the first channel. The degradation of the signal can be manifested by an increase in the bit error rate of the signal on the first channel.
The fault signal is typically sent as part of the frame overhead for the protocol used on the ring. For example, in the case of a SONET ring, the fault signal is encoded on either the V4 byte or the Z4 byte.